Python: sorted() vs list.sort()

Summary

其实这两个函数的区别看 help 就能看出很多：

>>> # Python 3.5.2
>>> help(sorted)
Help on built-in function sorted in module builtins:

sorted(iterable, key=None, reverse=False)
    Return a new list containing all items from the iterable in ascending order.
    
    A custom key function can be supplied to customise the sort order, and the
    reverse flag can be set to request the result in descending order.

>>> # Python 3.5.2
>>> help(list.sort)
Help on method_descriptor:

sort(...)
    L.sort(key=None, reverse=False) -> None -- stable sort *IN PLACE*

• sorted() 的意思更像是 sorted_list_from(x)
  • 它可以用在任意的 iterable 上，比如：
    • string
    • tuple
      • 虽然 tuple 是 immutable 的，但 sorted() 又不是 in-place 操作，所以无妨
    • dict
      • 返回的是 sorted keys
    • generator
  • 返回值一定是个新 list
• sort() 竟然是 list 类型专属的 method，有点出乎我的意料
  • 它是 in-place 操作
  • 号称是 stable 的但是我没有研究它具体是怎么实现的

这两个函数的 key 参数的用法是一样的，都应该是一个 function，表示用 key(x1) < key(x2) 而不是 x1 < x2 的关系来决定 x1, x2 的顺序。key = None 是应该是默认使用 x1.__lt__(x2)

Recipe 1: How to sort dict entries by keys?

这个 topic 有点微妙，因为我们一直强调的是 “dict 并不能保证 key 的 insertion order”，但这一点随着 python 的发展在发生变化。根据 Eugene Yarmash:

• Python 3.7+
  • In Python 3.7.0 the insertion-order preservation nature of dict objects has been declared to be an official part of the Python language spec. Therefore, you can depend on it.
• Python 3.6 (CPython)
  • As of Python 3.6, for the CPython implementation of Python, dictionaries maintain insertion order by default. This is considered an implementation detail though; you should still use collections.OrderedDict if you want insertion ordering that’s guaranteed across other implementations of Python.
• Python >=2.7 and <3.6
  • Use the collections.OrderedDict class when you need a dict that remembers the order of items inserted.

所以需要根据你使用的版本来决定你是该用 dict 还是 OrderedDict:

# Python 3.5.2

x = {"cranberry": 99, "durian": 0, "apple":2, "banana": 3}

sorted_x = OrderedDict((key, x[key]) for key in sorted(x))
    # OR
sorted_x = OrderedDict(sorted(x.items()))  # Better!

注意 OrderedDict 的 __str__ 和 __repr__ 打印出来的 representation 和普通的 dict 不一样

>>> x
{'apple': 2, 'banana': 3, 'cranberry': 99, 'durian': 0}
>>> sorted_x
OrderedDict([('apple', 2), ('banana', 3), ('cranberry', 99), ('durian', 0)])

Recipe 2: How to sort a list of custom objects?

如果你有可以通用的 __lt__(self, other) 逻辑，可以去实现 __lt__(self, other)。但要注意不要专门为了某一个特定的 sorting 的需求去写 __lt__(self, other)。

class Student:
    def __init__(self, name, grade, age):
        self.name = name
        self.grade = grade
    
    def __lt__(self, other):
        return self.grade < other.grade  # Is this always the case? 

更 general 的、不那么 invasive 的写法是用 key 参数:

sorted(student_objects, key=lambda student: student.grade)

from operator import itemgetter, attrgetter

sorted(student_tuples, key=itemgetter(1))
sorted(student_objects, key=attrgetter('grade'))